1. Cross Reference to Related Application
This application for patent is related to prior copending application U.S. Ser. No. 449,756, now U.S. Pat. No. 4,469,917, filing date 12-14-82, Synchronized Data Transmission System.
2. Field of the Invention
The present invention relates generally to telephone data transmission systems, and more particularly to a time independent data transmission system which allows a central computer, for example, to access at any time a remote data set, such as an electronic publication storage device, a hybrid telephone-cable television polling system, or a computer data base subscriber's data set, via non-dedicated telephone lines and communicate with that data set without interfering with normal use of the subscriber's telephone. The present invention finds application in many areas, one of which, for instance, might be a shop-by-computer system where it might be necessary to transmit information to and from an individual subscriber's data set connected to an information storage device. Another might be the field of power demand data retrieval, wherein a central computer accesses a remote data set which has stored therein data corresponding to the electrical or other power consumed at the remote location. Still another might be remote control of heating, ventilating and air conditioning equipment through telephone lines. Although the invention will be described for use with telephone lines, it will be apparent to one skilled in the art that the invention can also be used with transmission media other than telephone lines.
3. Description of the Prior Art
Various systems are presently known which allow central computers to access individual subscriber's data sets via dedicated telephone lines. For example, see U.S. Pat. Nos. 4,126,762, 4,104,486, 4,394,540 and 4,345,113. In the U.S. Pat. Nos. 4,104,486 and 4,394,540 patents, the telephone is isolated from the telephone line during a selected time window. In the U.S. Pat. No. 4,126,762 patent, in one embodiment, a time window is utilized. In another embodiment, sequential ringing signals, having a different time spacing than signals from a normal telephone call condition the remote terminal to intercept the ringing signals and to provide selective connection of a data accumulator to the telephone line. It is preferable to use the subscriber's existing telephone line rather than install a separate dedicated line to provide access to the data set. Thus, systems of this type require that the telephone line be available for both use by ordinary persons and by computers calling the subscriber. Requirements of systems of this type are that when computer calls are received the call is automatically transferred to a data set and the telephone preferably should not ring.
In applicant's prior U.S. Pat. No. 4,345,113, an automatic telephone message interception system is described which permits the use of a single telephone line for both ordinary personal use and for receiving calls from a computer, wherein computer calls are automatically transferred to the subscriber's data set without ringing the subscriber's telephone. An ordinary (non-computer) call made to the subscriber, would, however, be answered by the automatic telephone message interception system described in this U.S. patent even if no one were present at the subscriber's telephone. The caller would thus be charged for a brief call even though no one answered the phone.
The subscriber could, of course, solve this problem by shutting off the automatic system if the telephone is to be left unattended, but the data set could not be accessed by the computer while the system was shut off.
The need thus existed for a system which both allows a central computer to access the subscriber's data set via a non-dedicated telephone line and which allows normal operation of the telephone by both the subscriber and other persons calling the subscriber. In particular, such a system should allow a computer to access the subscriber's data set without ringing the subscriber's telephone and should allow persons to call the subscriber at any time without being charged for the call if the telephone is unanswered.
In applicant's prior copending application U.S. Ser. No. 449,756 and filing date 12-14-82, an apparatus which satisfies the above need is described.
In that patent application, however, a system and apparatus is described for connecting a central computer to a remote subscriber's data set using the subscriber's non-dedicated telephone line during a preselected window in time. In the present application, the apparatus described in that patent application has been modified to permit connection of the central computer at any time to a remote subscriber's data set using the subscriber's non-dedicated telehone line and without interference of any sort with the normal use of the subscriber's telephone. In the system described in the above patent application Ser. No. 449,756, and filing date 12-14-82, the apparatus detects a ringing signal on the telephone line during a preselected window in time during which the subscriber's data set is connected to the telephone line and the subscriber's telephone instrument is disconnected. The apparatus does nothing until the ringing signal stops, indicating that the caller has hung up. The apparatus then causes the data set to dial up the central computer and proceed to transmit or receive data during the time window.
Accordingly, the need exists for a system which allows a central computer to access a subscriber's data set via non-dedicated telephone lines at any time (i.e., no limitation to a time window). Furthermore, such a system should allow normal operation of the telephone by the subscriber without interference in any way with the subscriber's use of the telephone at any time, i.e., the subscriber's telephone should not ring when the central computer calls, the subscriber should be allowed to use the telephone even if data transmission with the computer is occurring and outside callers should not be charged for calls when the phone is unanswered, for example.